Implement an optional compiler bootstrapping phase

.gitlab-ci.yml

bin/generate-gitlab-ci-yml.sh

bin/rebuild-package.sh

lib/spack/docs/example_files/spack.yaml

lib/spack/spack/binary_distribution.py

lib/spack/spack/cmd/buildcache.py

lib/spack/spack/cmd/release_jobs.py

lib/spack/spack/schema/gitlab_ci.py

lib/spack/spack/schema/specs_deps.py

lib/spack/spack/test/cmd/release_jobs.py